The present invention relates to a semiconductor laser diode and, more specifically, the present invention relates to a semiconductor laser diode having excellent temperature characteristics.
Compound semiconductors have generally been utilized as materials for semiconductor laser diodes in recent years. Then, improvements have been conducted over and over for semiconductor laser diodes seeking for more stable lateral mode and higher temperature operation or the like.
In a basic structure of a laser diode, as shown in FIG. 5, a first clad layer 4, an active layer 2 and a second clad layer 5 are fabricated in this order on a substrate 9. As an example of a modification for the lateral structure of the basic laser diode, shown in FIG. 6, having a contact layer 7 capped at an upper portion, a structure having a current blocking layer (buried layer) 6 for improving the LD characteristics has been known. Further, as examples of modification for the vertical structure of the basic laser diode, there have been known a laser diode having a optical guide layer 2 (LOC structure) as shown in FIG. 7, a laser diode having a optical guide layer disposed on both sides (SCH structure) and a laser diode in which the composition of a optical guide layer is gradually varied in order to change the refractive index continuously (GRIN-SCH structure).
Further, the modification of the active layer itself in which the active layer is constituted as a quantum well structure such as a single quantum well (SQW) structure or a multi quantum well (MQW) structure have also be known.
These laser diodes have been obtained generally, for example, by a liquid phase epitaxy (LPE) method, a molecular beam epitaxy (MBE) method and a metallo-organic chemical vapor deposition (MOCVD) method, and the development, in particular, in the MBE method and the MOCVD method enables to produce a thin layer structure at an order of one monolayer.
Various proposals have been made for a structure vertical to the substrate in a laser diode as described above, but there are the following actual problems in the preparation of the layer structure thereof. Referring, for example, to MBE growth of an InGaAs system material on a GaAs substrate which has been expected as a light emitting material for a 980 nm region required for an exciting light source of an optical fiber amplifier in recent years, various structure in the laser diode using AlGaAs as a clad layer, GaAs as an optical guide layer and InGaAs as an active layer have been adopted for the vertical structure deposited onto the substrate. However, when a laser diode having this structure is produced, since an optimum growth temperature between AlGaAs and InGaAs has a temperature difference of about 200.degree. to 250.degree. C., there should has been conducted a method, for example, of performing the growth interruption and of lowering the temperature of the substrate after growth of the GaAs guide layer. However, by performing the growth interruption, since the crystal surface thereof is allowed to stand for a long time in preparation atmosphere, the contamination thereof is brought about, thereby causing deterioration of quality of the crystal. This is a fatal problem to the laser diode. In addition, in a structure in the laser diode using AlGaAs as a clad layer, AlGaAs having an Al concentration relatively lower than that of the clad layer as a guide layer and InGaAs as an active layer, during the growth interruption and lowering the temperature of the substrate after growth of the optical guide layer, the deposited material passes through a temperature region (for example, 600.degree. to 630.degree. C.) that an AlGaAs bulk crystal surface morphology in the optical guide layer does not show a mirror surface. Accordingly, it is theoretically expected that use of AlGaAs system for the optical guide layer can increase the quantum efficiency of spontaneous emission as compared with the case of using the GaAs system for the optical guide layer since carriers can be confined more effectively. However, since the deposited material passes through such a temperature region that the AlGaAs bulk crystal surface morphology does not show a mirror surface, there arises a phenomenon that the quantum efficiency of the obtained laser diode is lowered. As described above, also in the modification of the vertical structure proposed so far, it has been experienced that characteristics of the materials can not fully be displayed depending on the system.
Further, the temperature characteristics of the semiconductor laser diode depends not only on the materials described above but also greatly on the vertical layer structure. In most of semiconductor laser diodes having a buried structure, no higher temperature characteristics than that determined by physical properties thereof such as a band offset can be expected since both of the clad layer and the optical guide layer have a simple bulk structure, so that the characteristic temperature (To) is about 180K for the AlGaAs system and about 90K for the InGaAs system. It can not be said such characteristic temperature is satisfactory for the semiconductor laser diode which is used under conditions that becomes severe more and more.
Further, it is difficult by existent crystal growth techniques such as the LPE method or the MOCVD method to selectively bury and grow a layer having a refractive index relatively lower to each of deposited layers in order to increase the light confinement factor not only vertical but also in parallel with the layer. In particular, in the AlGaAs system semiconductor laser diodes, although a semiconductor layer having a refractive index relatively lower to the active layer can be formed as a buried layer, a semiconductor layer having a refractive index relatively lower than the clad layer can not be formed as a buried layer.
In view of the above, there has proposed as the modification of the LOC structure such a light-confining structure as shown in FIG. 8, in which the lateral current blocking layer 6 having lower refractive index relative to that of an active layer 1, a optical guide layer 2 and clad layers 4, 5 is fabricated (U.S. Pat. No. 5,355,383).
However, further higher performance has been demanded for the laser diode and a development for a semiconductor laser diode having better temperature characteristics has been demanded.
As a result of the present inventors' earnest studies, it has been found that by forming successively on the substrate at least a first clad layer, an active layer and a second clad layer, wherein at least one layer adjacent to the active layer, for example, at least one of the first clad layer and second clad layer has at least one superlattice in the direction parallel with the substrate, forming a buried layer for current blocking at both lateral sides of cavity direction of the active layer, and selecting the composition of the first clad layer and the second clad layer such that the (average) refractive index of the first clad layer and the second clad layer is less than a (average) refractive index of the active layer, the obtained semiconductor laser diode has more excellent temperature characteristics, is capable of satisfactory light confinement both in the vertical direction and the horizontal direction to the layer and is suitable to high temperature operation at low threshold current. The present invention has been attained based on the finding.